Electrolytic cell for the production of aluminum

ABSTRACT

ELECTROLYTIC CELL FOR THE PRODUCTION OF ALUMINUM HAVING PRESCRIBED MAXIMUM HORIZONTAL DISTANCE BETWEEN OUTER LOWER EDGES OF THE ANODES AND ADJACENT WALL SURFACES OF THE POT LINING, POT LINING WITH SPECIFIED THERMAL ISOLATING POWER, CATHODE BARS INSULATED BENEATH THE SIDE WALL OF THE LINING AND SPECIFIC RATIO OF IRON CROSS SECTION TO CARBON CROSS SECTION THROUGH THE BOTTOM OF THE LINING, IN ORDER TO REDUCE THE HORIZNTAL COMPONENTS OF THE ELECTRIC CURRENT IN THE CELL.

April 1973 w. SCHMIDT HATTING 3,

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ELECTROLYTIC CELL FOR THE PRODUCTION OF ALUMINUM Filed Aug. 25, 1971 4Sheets-Sheet 3 Fig.5

April 17, 1973' w, SCHMIDT HA TING 3,728,243

ELECTROLYTIC CELL FOR THE PRODUCTION OF ALUMINUM Filed Aug. 25, 1971 v 4Sheets-Sheet 4.

Fig.

United States Patent 3,728,243 ELECTROLYTIC CELL FOR THE PRODUCTION 0FALUMINUM Wolfgang Schmidt-Hatting, Chippis, Switzerland, assignor toSwiss Aluminium Ltd., Chippis, Switzerland Filed Aug. 25, 1971, Ser. No.174,892

Claims priority, application Switzerland, Sept. 1, 1970,

13,101/70 Int. Cl. C22d 3/02, 3/12 US. Cl. 204-243 R 2 Claims ABSTRACTOF THE DISCLOSURE To obtain aluminium by electrolysis, aluminium oxide(A1 0 alumina) is dissolved in a fluoride melt. Electrolysis is carriedout in a temperature range of about 940 to 975 C. The cathodicallydeposited aluminium collects under the fluoride melt on the bottom ofthe cell. Anodes of amorphous carbon are dipped from above into themelt. The electrolytic decomposition of the alumina causes oxygen toform on the anodes, and this combines with the carbon of the anodes toform CO and CO A typical aluminium electrolysis cell is showndiagrammatically in FIGS. 1 and 2 of the accompanying drawings, whichare a longitudinal section and a transverse section.

A fluoride melt (the electrolyte) is contained in a steel pot in whichis a layer of insulation 13 and a carbon lining 11. The insulation 13 isof refractory thermally insulating material. Cathodically precipitatedaluminium 14 collects on the bottom 15 of the cell. The surface 16 ofthe liquid aluminium acts as the cathode. Iron cathode bars 17 areembedded in the bottom of the carbon lining 11 and serve to conductcurrent from the bottom of the cell to the exterior.

Anodes 18 of amorphous carbon dip into the fluoride melt from above soas to conduct the direct current to the electrolyte. They are fixed byrods 19 and clamps 20 to two anode bus bars 21. These togetherconstitute an anode beam. The electrolyte 10 is covered with a crust 22of solidified melt and on top of this is a layer 23 of alumina.

The distance d from the underside 24 of the anode to the upper surface16 of the aluminium (also called interpolar distance) can be varied byraising or lowering of the anode beam 21, 21 with the aid of the liftingmechanisms 25 which are mounted on columns 26. As a result of attack bythe oxygen liberated in the electrolysis, the anodes are consumed ontheir underside to an extent of about 1.5 to 2 cm. each day, accordingto the particular construction of the cell.

The cathode bars 17 have two tasks. They collect the current from theactive part of the carbon bottom beneath the anodes 18 and they conductit out of the cell. Where they collect the current and are designated by29, the current intensity in the cathode bar increases on both sidestowards the exterior. At 28, outside the active part 27 of the carbonbottom, the cathode bars serve as pure current conductors. From eachcell, cathode bus bars 30 conduct the current from terminals at theouter ends of the cathode bars 17, to the anode beam 21, 21 of thefollowing cell.

3,728,243 Patented Apr. 17, 1973 To reduce heat losses here, thecross-section of the iron cathode bars is reduced outside the activepart 27 of the carbon bottom. Thus the flow of heat out of the meltthrough the bars to the exterior is reduced. This reduction is thesubject of our patent application No. 139.154 Apr. 30, 1971.

The electrolyte possesses a substantially poorer electric conductivitythan the liquid aluminium which is situated on the bottom of the cell.The ratio of the two conductivities is between l0 :1 and 10- 1. If thecurrent withdrawal through the carbon bottom does not locally exactlycorrespond to the current feed through the anodes into the electrolyte,horizontal current density components must occur in the melt, which arecaused by the local difference between feed and withdrawal of thecurrent. The great difference of the electrical conductivties of the twoStratified liquids has the effect, according to the tangent law ofelectric current dynamics, that a kink occurs in the current lines atthe boundary surface between electrolyte and liquid aluminium. Theresult is that the current lines in the electrolyte, to a firstapproximation, extend vertically. On the other hand in the metal majorhori zontal current density components can occur which can be locallygreater than the vertical. The different current density components inthe electrolyte and in the liquid aluminium, in cooperation with themagnetic induction between the two media, result in differences in thepressure which can be compensated only by a doming up of metal. This canamount to many centimetres in height since the domed-up metal is coveredby the electrolyte and thus has only an effective specific gravitycorresponding to the difference of density between electrolyte andmetal.

Furthermore, the horizontal current density components in cooperationwith the magnetic induction can cause a force field distribution in theliquid metal which is not free from rotation. The consequence of this isa flow of metal combined with a major doming up of metal, which in turnis caused by current density components induced by this movement of acurrent conductor in a magnet field. Doming up and movement of metal aredetrimental to the electrolytic efficiency (ratio of the quantity ofaluminium actually obtained to the quantity theoretically precipitatedaccording to Faraday). If the electrolyte efiiciency falls, the electricenergy consumption rises (kWh/kg. Al).

If therefore only vertical current density components are present in themetal and in the melt, a doming up of metal without metal movement isimpossible. Nevertheless, there may be rotation drive in the metal, asshown by the following equation of the volume forces k:

ii 5 6B, 6B, 6B, 6y By+ 6a 6x 6y 62 Here j j and i signify the currentdensity components in the metal in the three axial directions and B Band B the corresponding components of the magnetic induction.

If it is ensured that the current withdrawal through the carbon bottomat the underside of the liquid metal corresponds to the current feed atthe upper side of the.

metal, the following components are zero: i and j and thus also thethree partial derivatives of j Only the last member of the rotary drivemust be caused to disappear by making eB /fi become little or zero,since i is always present (normal electrolytic current).

Normally, horizontal current density components occur in both axialdirections. In the case of incorrect dimensioning of the thermalinsulation on the walls of the cell, a direct flow of current from theanodes to the cell rim is possible, which generates horizontal currentdensity components.

The component transverse to the cell longitudinal axis may be augmenteddue to the fact that the cathode area, often on account of anexcessively great distance between the outside of the anode and thecarbon lining of the side walls, is larger than the anode area. Moreoverthe iron cathode bars outside the active part of the carbon bottom canalso take up current, if there they are not sufiiciently electricallyinsulated from the carbon lining of the walls of the cell. Moreover, ifthe cross section of the iron cathode bars in the active part of thecarbon bottom is too small, a great outward displacement of theelectrolytic current takes place in the carbon bottom, likewisegenerating powerful horizontal current density components.

There is also the fact that due to incorrect dimensioning of thecross-sections of the cathode bus bars which conduct the current fromone cell to the following cell of the series, powerful horizontalcurrent density components can occur in the liquid aluminium which, insome cases, are locally greater than the vertical.

We have faced the problem of largely suppressing the horizontal currentdensity components in an aluminium electrolytic cell for a currentintensity of 70 ka. and above.

On the basis of extensive experiments we have developed cells which havea series of inventive features which are described below and thensummarised and which in conjunction ensure success.

One feature is that the horizontal distance between the outer loweredges of the anodes and the inner faces of the walls of the steel potdoes not exceed '55 to 60 cm. If 20 cm. are deducted for thermalinsulation and carbon lining, a horizontal interval of at most 40 cm.remains between the outer lower edge of the anodes and the furnace rim,that is the inner surface of the walls of the carbon lining. The minimumhorizontal distance between the outer lower edge of the anodes and thefurnace rim is 25 to 30 cm.

A second feature is that the thermal resistance of the walls of theinsulation 13, between the walls of the carbon lining 11 and the wallsof the steel pot 12, lies between 0.5 and and As a consequence, a solidlateral cryolite crust is formed by removal of heat which reduces thecathodic, currentcollecting aluminium area and effectively limits thelateral current flow into the furnace rim.

A third feature is that the parts of the cathode bars beneath the sidewall of the carbon lining are surrounded by insulation. This is shown inFIGS. 3 and 4 of the accompanying drawings which are a diagrammaticlongitudinal section and transverse section of a cell embodying thefeatures of the present invention.

The cathode bars have their lower faces flush with the interface of thebottom of the carbon lining and the bottom of the insulation. Thus thereis no carbon beneath the bars. Furthermore, outside the active part 27of the carbon bottom the bars are surrounded with insulation 31. Thusthere can be no current flow into the bars outside the active part ofthe carbon bottom. We recognise that this feature per se has beenproposed previously.

The outward current displacement cannot be entirely avoided since thecell bottom (carbon and cathode bars) has a substantially poorerelectrical conductivity than the liquid aluminium situated above it. Afourth feature is to place into the active part of the carbon bottom thelargest cathode bar cross-section 29 which is compatible with mechanicalstrength of the carbon bottom. The ratio of iron to carbon should amountto at least 17:100and at most 20:100. If a smaller iron cross-section isprovided, unacceptably high horizontal current density components occurin the liquid aluminium. If, on the other hand, a greater iron crosssection is provided, a mechanical weakening of the carbon lining occurs,this weakening being caused by the larger thermal coeflicient ofexpansion of iron in comparison with that of carbon.

To summarise: according to this invention an electrolytic cell for theproduction of aluminium by electrolysis of alumina in a melt, comprisesa pot body of steel; a layer of thermal and electrical insulationagainst the inside of the body, a lining of carbon against the inside ofthe insulation layer; the body, the layer and the lining each consistingof a bottom, two side walls and two end walls; iron cathode bars eachhaving at least a part within the lining and a part passing through aside wall of the insulation layer and through a side wall of wall of theinsulation layer and through a side Wall of the body; and anodesarranged to dip into electrolyte in the pot; wherein the horizontaldistance between outer lower edges of the anodes and adjacent wallsurfaces of the lining does not exceed 40 cm.; and thermal resistance ofthe walls of the insulation layer is between C h. C.

' and 1X10 the parts of the cathode bars beneath the side wall of thelining are surrounded by insulation; and the ratio of iron cross sectionto carbon cross section in any vertical plane from end to end of thecell through the bottom of the lining is between 10:100 and 201100.

By means of the invention transverse horizontal current densitycomponents are largely suppressed, and longitudinal components arereduced.

No continuous iron current conductors are present in the longitudinaldirection of the cell. Neverthless, substantial horizontal currentdensity components can persist in the cell longitudinal direction in theliquid aluminum unless by suitable dimensioning of the cathode barswhich conduct the current from the one cell to the anodes of thefollowing cell of the series it is ensured that each cathode bar of thecell bottom as far as possible carries the same current.

This can be achieved by a circuit arrangement which is illustrated inFIG. 6 of the accompanying drawings and which is per se the subject ofmy co-pending application Ser. No. 174,890 filed Aug. 25, 1971(corresponding to Swiss appl. No. 13,100/70). According to thisarrangement a plant comprises a plurality of electrolytic cells; eachcell including at least one terminal outside the pot on each cathodebar, and an anode beam carrying the anodes, and electrical connectingmeans comprising a plurality of cathode bus bars each of which connectsa respective group of at least one of the cathode bar terminals of onecell to the anode beam of the next cell, the cross sections of theindividual bus bars being such that, when an equal current flows througheach cathode bar,

- then the voltage drop is the same along each bus bar from therespective bar terminal nearest to the anode beam to a point midwayalong the anode beam.

FIG. 6 shows a resistance substitute circuit diagram calculated from theliquid aluminium of one cell to the middle M of the anode beam of thefollowing cell.

R is the proportional bottom resistance for an iron cathode bar,calculated from the liquid aluminium to the outer end of the cathodebar.

A first cathode bus bar collects the current from n cathode barterminals. To the commencement of the anode beam of the following cell,it has the resistance R Analogously a second cathode bus bar with itsown resistance R collects the current from n terminals, a third cathodebus bar with its own resistance R the current from n;, terminals and soon. R is the resistance of the anode beam of the following cell,calculated to the middle M. of the anode beam.

I is the total cell current.

Each cathode bar should conduct the same current 1 No horizontal currentdensity components occur in the longitudinal direction of the cell inthe liquid aluminium if the cross sections of the individual bus barsare so chosen that the voltage drop in each cathode bus bar, from thepoint of feed to the last iron cathode bar (points A, B, C, etc.) to themiddle M of the anode beam of the following cell is the same. In thiscase in the first bus bar a current 11 1 flows, in the second bus bar acurrent 11 1 in the third bus bar a current 11 1 and so on. Thecalculation must take place as if the current I from the anode beam ofthe following cell were not tapped continuously but at a point exactlyin the middle of the cell (point M).

FIG. 5 of the accompanying drawings is a diagrammatic plan of an actuallayout. It shows a series of three cells A, B, C. In this example eachcell includes three groups D, E, F of iron cathode bars on each side.Each group comprises three iron cathode bars G, H, I and a respectivebus bar K. In this example two bus bars K are connected to the left endof the anode beam and one bus bar K to the right end. L denotes thedirection of the pot line current.

A complete series comprises from a few cells up to 100 or more. At thefirst cell of the series the invention is only to be applied to the busbar connection to the second cell. At the end of the series all bus barsare connected together.

The number of the iron cathode bars depends on the size of the cell, onthe current intensity and on several other factors; for example a100,000 ampere cell can include between and 20 cathode bars (meaningbetween 10 and 20 protruding ends on each side; often the cathode barsare divided in the middle of the carbon bottom, that is to say that thetwo halves are disposed in such a way that they have a common axis butdo not touch eah other). As to the number of bus bars, there are manypossibilities from one bus bar for each cathode bar to only one bus barfor all cathode bars together on each side.

In FIG. 5 the cells are end to end. They may alternatively be side byside.

The anode beam can consist of one or more single anodic bus bars. In theFIGS. 2 and 4 the anode beam 21 consists of two anodic bus bars.

I claim:

1. An electrolytic cell for the production of aluminium by electrolysisof alumina in a melt, comprising:

a pot body of steel,

a layer of thermal and electrical insulation against the inside of thebody, a lining of carbon against the inside of the insualtion layer,

the body, the layer and the lining each consisting of a bottom, two sidewalls and two end walls,

iron cathode bars each having at least a part within the bottom of thelining, a part beneath a side wall of the lining and a part passingthrough a side wall of the insulation layer and through a side wall ofthe body,

and anodes arranged to dip into electrolyte in the pot,

wherein the horizontal distance between outer lower edges of the anodesand adjacent Wall surfaces of the lining does not exceed 40 cm.,

the thermal resistance of the walls of the insulation layer is betweenh. C. h. C.

kcal. kcal.

the parts of the cathode bars beneath the side wall of the lining aresurrounded by insulation,

and the ratio of iron cross section to carbon cross section in anyvertical plane from end to end of the cell through the bottom of thelining is between 17:100 and 20:100.

2. A plant comprising a plurality of cells according to claim 1 inseries, each cell including at least one terminal outside the pot oneach cathode bar, and an anode beam carrying the anodes, and electricalconnecting means between each cell and the next in the series, eachconnecting means comprising a plurality of cathode bus bars each ofwhich connects a respective group of at least one of the cathode barterminals of one cell to the anode beam of the next cell, the crosssections of the individual bus bars being such that, when an equalcurrent flows through each cathode bar, then the voltage drop is thesame along each bus bar from the respective bar terminal nearest to theanode beam to a point midway along the anode beam.

0.5)(10 and 1 X10 References Cited UNITED STATES PATENTS 2,786,0243/1957 Wleugel 204243 R 3,562,136 2/1971 DeVarda et a1. 204243 R3,607,685 9/1971 Johnson 20467 3,649,480 3/1972 Johnson 204243 R HOWARDS. WILLIAMS, Primary Examiner D. R. VALENTINE, Assistant Examiner US.01. X.R. 204-244

